Negotiators for the Society of Professional Engineering Employees in Aerospace have “extracted everything we could” from Boeing without calling for a strike, they wrote in a letter sent Thursday to members. Technical workers will have until 5 p.m. on March 18th to return their ballots to the union.

Boeing was unwilling to improve its offer due to the narrow margin of rejection by technical workers, SPEEA leaders wrote. Following the vote, the two sides resumed negotiations for less than a day before SPEEA leaders decided to send the contract back to technical workers for a vote.

“The membership must decide if it is willing to engage in a potentially lengthy strike in an effort to achieve a better contract offer,” union leaders wrote.

Boeing’s offer includes 5 percent annual increases to wage pools over the four years of the contract. Medical benefits remain the same without additional contributions by employees.

The major change is to the retirement plan for new SPEEA members, who will be enrolled in a 401(k) plan rather than the defined pension. After engineers accepted Boeing’s contract, SPEEA President Tom McCarty deemed the fight for pension “dead.”

That’s the sentiment Boeing has been pushing from the beginning of negotiations, saying it needed to cut pension costs to stay competitive. In recent months, Boeing appealed to SPEEA members’ sense of loyalty as the the company strives to return its grounded 787 to commercial service.

“We urge employees to take charge of their future and approve the contract. It is time for all of us to come together and focus on the challenges facing the company,” Boeing said in a statement on Thursday.

SPEEA leaders have said a strike by technical workers would still hurt Boeing even though engineers will not be taking part. Technical workers interface between engineers and machinists, who build Boeing jets. The technical workers also play a role in flight testing, customer service and calibration.